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A hash after a move, recording that the move ends the game.
Also known as: Chess checkmate symbol, Mate symbol, Chess hash, Double plus (++)
A hash appended to a move means checkmate: the king is in check and no legal move escapes it. The game is over at that move. Some publications write ++ or mate instead, which is a typographic choice rather than a different claim.
Records that the move delivers checkmate and the game ends. The FIDE Laws define checkmate as the position and the notation that records it.
Written as # in most modern publications and as ++ in older or continental ones. Both mean checkmate, and no meaning turns on which is used.
A hash — two slanted vertical strokes crossed by two horizontal ones — written immediately after the move it describes.
The plus sign means the king is attacked and must be defended; the hash means it is attacked and cannot be. Only the second ends the game.